Underwater grasses expanded their coverage to almost 90,000 acres of the Chesapeake in 2002, their greatest showing since annual surveys began nearly two decades ago and almost half of the Bay Program’s restoration goal.
Much of the improvement was driven by last year’s drought, which resulted in some of the clearest water seen in the Chesapeake in decades, allowing sunlight to reach the important plants so they could thrive.
“2002 was a good year in general,” said Bob Orth, a researcher with the Virginia Institute of Marine Science who oversees the annual survey. “The very rapid changes that we see on a year-to-year basis continues to reinforce how quickly these plants can rebound if they have good water quality.”
Last year’s gains, though, will likely be offset by losses this year as higher-than-normal river flows flooded many grass beds with sediment and nutrients. Extensive diebacks have been observed, especially in the mid-Bay area. “This very very wet year is going to have a big impact on the numbers,” Orth noted.
Figures from this year will not be available until the middle of 2004.
But the 2002 figures, released in September, showed 89,658 acres of submerged aquatic vegetation—or SAV—in the Bay last year. That was an increase of more than 30 percent over the 69,126 acres observed in 2000.
Baywide figures were up sharply from 2001 as well, but direct comparisons between 2001 and last year are difficult in many areas because flight restrictions immediately after the Sept. 11 terrorist attacks prevented the survey from being completed.
Last year was the second year of a severe drought, and the fourth straight year of lower-than-normal river flows into the Chesapeake. Dry conditions result in fewer nutrients, which cause algae blooms, and sediments, which cloud the water entering the Bay.
“We’re clearly seeing the results of two straight years of drought,” said Mike Naylor, of the Maryland Department of Natural Resources and the chair of the Bay Program’s SAV Workgroup.
“But it’s not just the drought,” he added. Naylor believes the huge amounts of seeds created by underwater plants in recent favorable years may have built up large “seed banks” in the sediment which made a rapid expansion possible during the good conditions produced during the drought.
Further, he said huge amount of seeds produced during 2002 should help the grass beds bounce back from this year’s poor conditions. “I won’t be worried if the numbers are way down in 2003,” he said. “The seeds are there now.”
Although the drought produced a Baywide increase, it also took a toll in places: Low flows resulted in higher than normal salinities that killed freshwater grass species in the upper parts of tidal tributaries.
That was evident in a regional breakdown of the survey:
- In the Upper Bay, which extends from the Susquehanna River to the Bay Bridge, the survey observed 13,166 acres last year, an 11 percent decrease from the 14,814 acres of grasses mapped in 2000. Grasses in that area appeared to have increased in 2001, but then dropped back this year, possibly because of higher salinities.
- In the Middle Bay, which stretches from the Bay Bridge to the Rappahannock River and Pocomoke Sound (including the Potomac River), grass coverage jumped to 52,937 acres last year, an increase of 58 percent from the 33,465 acres observed in 2000.
- In the Lower Bay, 23,507 acres were observed last year, a 13 percent increase from the 23,519 acres seen in 2000.
The overall 2002 increase was driven by a huge surge in the middle part of the Chesapeake, which is dominated by widgeon grass, a species notorious for widely fluctuating from year to year. That area appears to have suffered the greatest setbacks from this year’s high flows.
Particularly encouraging in the 2002 figures, Orth said, was the continued comeback on the Susquehanna flats, an extensive underwater grass bed in the upper Chesapeake that historically was an important wintering area for migratory waterfowl. The beds were nearly obliterated after Hurricane Agnes in June 1972, which sent huge flows of sediment into the Bay.
“The density and size of that bed is just phenomenal,” Orth said. “When we first started mapping the SAVs on a Baywide basis you could barely see a patch of grass on the flats.”
Orth said the freshwater grass beds survived in Susquehanna flats because flows from the nearby Susquehanna River were probably enough to keep salinity levels low.
Expanding grass beds has long been a major Bay Program objective because they are considered crucial habitat for many Chesapeake Bay species. They provide food and shelter for juvenile fish, clams and crabs—densities of young blue crabs can be 30 times greater in grass beds than in unvegetated areas. Grasses are also important food for waterfowl.
Today’s grass beds cover only a fraction of the area once blanketed by the Bay’s “underwater meadows” that often grew at water depths of more than 6 feet in places.
In recent decades, huge increases in nutrient and sediment runoff have clouded the water, blocking essential light from reaching the plants, which today are only rarely seen in deep water. That resulted in massive declines from historic levels. When the annual survey began in 1984, only 37,000 acres were present.
The Bay Program earlier this year set a 185,000-acre restoration goal for underwater grasses. The goal was based on an extensive review of historic aerial photographs that scientists used to determine the maximum amount of grass observed during any single year in each part of the Chesapeake.
The Bay states are in the process of adopting new water quality criteria that will help create water quality conditions needed to allow grasses to reach the goal. Huge nutrient and sediment reductions will be needed throughout the watershed to reach the new goals.
The annual Baywide grass estimate, made for the Bay Program by the Virginia Institute of Marine Science, is derived from the analysis of more than 2,000 black-and-white aerial photographs taken between May and October.